The automatic recognition of images by computerised devices is a very important problem in many fields. In one of its simplest forms, the problem includes being able to recognise the presence or absence of particular objects in an input image. The input image usually takes the form of a two dimensional array of pixels of a given resolution. The objects to be detected in the input image may be subject to varying degrees of rotation and/or scaling and may appear in any part of the image. As an example which will be examined in much greater detail hereinafter, it may be desired to recognise the presence or absence of a particular document in the input image.